The overweight and obesity epidemic is one of the greatest health crises humanity has ever faced. And it's feeding into the global environmental crisis too.

Gluttony, the dietary version of greed, is a word seldom used these days, which is odd. Ancient Romans and their vomitoria, upper-crust Edwardians and their elaborate six-course meals -- we make them look abstemious.

No-one has ever consumed as much, with such dire consequences, as us 21st century guzzlers.

A weight on the world
We're growing. According to the most recent World Health Organisation stats, obesity rates have more than doubled in the last 30 years. Over 1.4 billion adults are overweight; of these over 500 million are obese.

We’re familiar with the heavy toll this takes on health: overfeeding now kills more people than hunger. The associated economical burden is also well-recognised: obesity-associated illness places a huge strain on our health systems, and claims the lives of many in their most productive years.

But fat is also a green issue. Ingestion is the most literal expression of consumption, and consumption is something we do awfully well, especially in the developed world. We modern humans want our cake, and we want to eat it too – with immediate effect.

We want some more
Food production is a major contributor to climate change, accounting for as much as 29% of greenhouse gas emissions by some recent estimates. Food is the third largest contributor to the average US household's carbon footprint after driving and utilities. Bigger bodies require more food to maintain, thus increasing demand for higher food production, thus increasing greenhouse gas emissions.

According to a study published in the International Journal of Epidemiology, a "normal population" (in which 3.5% of people are classified as obese), requires 19% less food energy, and consequently produces fewer greenhouse gasses, than an overweight population (around 40% obesity, which is where several developed nations are headed).

Furthermore, it's estimated that up to a third of the food we buy gets wasted. Eating more food means buying more and thus wasting more. Most wasted food goes into landfills, where it decomposes and gives off methane, a potent greenhouse gas. Or, it is incinerated, which produces the greenhouse gas carbon dioxide, and toxic air pollutants.

A recent study in BMC Public Health, "The weight of nations: an estimation of adult human biomass", estimates that if everyone had the same average body mass index (BMI) as the USA, it hincrease in human biomass of 58 million tonnes would be equivalent in mass to an extra 935 million people of average body mass, and have energy requirements equivalent to that of 473 million adults. the increase in human biomass of 58 million tonnes would be equivalent in mass to an extra 935 million people of average body mass, and have energy requirements equivalent to that of 473 million adults. """,,""' estit would be equivalent, in terms of food energy demands, as having an extra half a billion people living on the earth.

When we worry about overpopulation, we need to worry not only about numbers of people, but their size too.

Overcoming inertia
It's not only feeding all those bigger bodies that contributes to greenhouse gas emissions, it’s having to shift them from place to place.

Transport already accounts for about 14% of global emissions, but overweight populations would demand even more in fuel energy use because it takes more energy to transport heavy people than light ones.

And Gluttony begets Sloth. Or, as the physicists put it: the greater the mass of an object, the greater its inertia, or "reluctance" to move. The more overweight we become, the more energy is needed to overcome our inertia, and the less we feel like moving. It's a lot harder for overweight people to get places by walking or cycling – or even to want to. And so we become more dependent on our cars. And on our planes and trains and lifts and escalators and remote-controls , all of which require more fossil fuels to run.

The six shades of gluttony
It’s clear from the abovementioned stats that packing unnecessary adipose tissue is not the best thing you can do for the planet. Even if you’re not clinically overweight, that extra belly-roll represents stored energy that took extra resources to provide, and demands extra resources to carry around.

Let's not forget though that there are multiple ways to over-exploit resources: you could be whip-thin from living off fancy, highly-processed diet foods and spending hours in resource-unfriendly gyms (and you may regularly drive to those gyms).

Or you could be overweight but eat primarily locally grown food and be a paragon of recycling and car-pooling.

It's also important to keep in mind that it's not just how much you eat, it's what you eat and how you eat it that can put a strain on resources.

In the Middle Ages, clerics distinguished between five or six different types of gluttony. In addition to the more familiar kind i.e. stuffing yourself, they recognised other forms of overindulgence with regard to food and drink, such as not having the patience to wait for mealtimes (it's called instant gratification now); lusting after expensive, exotic foods and seasonings; and placing too much importance on affairs of the palate.

The notion that you shouldn't enjoy your food too much is a bit antiquated, but some of these 12th century expressions of gluttony are worth looking at in modern-day environmental terms.

We don't just demand more food, we demand huge variety of foodstuffs, and in how these taste and look. Some of us are very fussy eaters indeed. We want Norwegian salmon when we live in Cape Town. Or Springbok venison when we live in Seoul. We turn our noses up at the slightest blemish on a perfectly sound apple.

The more elaborately processed, packaged and prepared a food item is, and the further it has to travel to reach our plate, the more damage it does to the environment. And that often goes for health and diet foods too, so don't think you're necessarily off the hook in this regard because you have a dainty BMI.

How to green your plate

Maintain a healthy body weight. Easier said than done, and much harder for some of us than others. For some people, all that's required is being strict about that mid-morning chocolate muffin, but for others it's a serious medical problem that's not about simply mustering up will-power. The causes of obesity are a poorly understood, complex mix of physiology, genetics and psychology, so don't go it alone. Get your doctor's advice on how best to go about dealing with an overweight problem.

Eat less meat, and go easy on the dairy products too. The livestock sector is estimated to account for 18% of greenhouse gas emissions, with beef production leading the field. Choose chicken or fish if you must eat meat. Eggs are another under-rated protein source. See: Which meat harms the planet most?

Eat less highly processed and packaged food. So more fresh food and bulk buying.

Waste not. Plan meals in advance so you only buy what you're going to use. If there is excess, don't just chuck it. Get into eating leftovers, and start a compost heap.

Buy local. It's helpful that foodstuffs trucked and flown from afar are expensive.

Learn to cook. You can be content with primarily vegetarian food if you know how to make it tasty.

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